Mercedes-Benz SLC 300
Modern turbocharged four-cylinder with direct injection and piezo injectors. Successor to the M271. Less susceptible to timing chain issues. Good balance of power and economy.
SLC 300: Lively Roadster
180 kW M274 in the SLC — finally enough power for genuine roadster fun. The M274 without timing chain worries and with adequate thrust makes the SLC a pleasure.
Engine Weaknesses 6
The high-pressure fuel pump on the M274 is driven by a lobe on the intake camshaft. Wear on the drive lobe or pump follower leads to pressure drop.
Symptoms: Metallic ticking from engine area, starting problems or extended cranking, power loss, fault code P0087
On the M274 too, the camshaft adjuster solenoids develop leaks. Oil penetrates the wiring harness by capillary action and can damage the engine ECU.
Symptoms: Fault code P0010, CEL, oil traces at camshaft connectors, lambda sensor failure, in severe cases ECU failure
Primary weak point of the M274: the electric thermostat sits deep in the engine block and is hard to reach. Replacement takes around 3 hours.
Symptoms: Engine does not reach operating temperature or overheats, temperature fluctuations, fault codes P0597–P0599
The PCV valve on the M274 clogs or breaks. Fault code P052E. Parts are inexpensive (~€63), but installed hidden under the engine ECU.
Symptoms: Fault code P052E, CEL, power loss, fault returns immediately after clearing
The M274 petrol engine has two NOx sensors that frequently fail. Each sensor costs around €600 as a part; with both sensors the repair bill quickly reaches €1,500.
Symptoms: Check engine light, NOx sensor fault code, elevated exhaust values, possible limp mode
The M274 with direct injection and no port injection tends to carbon deposits on intake valves like all GDI engines. Walnut blasting every 60,000–80,000 km is recommended.
Symptoms: Stumble under partial load, rough idle, slight power loss
Vehicle Weaknesses 6
Mercedes recalled SLK/SLC R172 between January and March 2016 due to incorrectly tightened inner track rod ends. Incorrectly torqued bolts could lead to loss of steering. Dealers inspected and replaced affected parts free of charge.
As with the predecessor SLK R171, the SLC R172 (facelift from 2016) also develops leaks at the vario roof when seals age or become brittle through UV exposure. The sealing of the metal folding roof in the area of the roof rails requires regular maintenance.
On the SLC R172, shock absorbers and suspension components are the primary weak point at higher mileages from around 80,000-100,000 km. The sportily tuned AMG Sport suspension wears faster than the standard setup.
The SLC R172 has increased quiescent current draw from numerous comfort and assistance systems. With frequent short trips or prolonged standing, the starter battery discharges, especially when used seasonally as a summer car.
On the SLC 220d, the 7G-Tronic automatic jerks uncomfortably during diesel particulate filter regeneration. The vehicle pulls briefly and abruptly -- particularly unpleasant in urban traffic. A software update can reduce this behaviour.
Like the predecessor SLK, SLC R172 owners also report noises from door cards and the rear bulkhead, which worsen at higher mileages. Temperature changes encourage plastic and rubber parts to expand.